This list of metro systems includes electrified rapid transit train systems worldwide. In some cases, metro systems are referred to as subways, U-Bahns or undergrounds. , 148 cities in 55 countries around the world host the approximately 160 metro systems that are listed here. The earliest metro system, the London Underground, first opened as an "underground railway" in 1863; its first electrified underground line opened in 1890, making the London Underground the world's first metro system. The metro system with the longest route length is the Shanghai Metro; the busiest one is the Beijing Subway; and the one with the most stations is the New York City Subway.